It is stated that the electric component of EM waves is stronger than that of the magnetic component.
That begs two questions:
1). what is the equivalency of the two components ie. how much magnetism is equivalent to a unit of the electric component or visa versa
2). and more importantly what is their exact ratio
The answer to these questions goes as follows, if I correctly understood it: for an electromagnetic plane wave the ratio between the electric and magnetid field is a constant of the given medium, known as the characteristic impedance of the medium (the modulus of it is the ratio between E and H, while the phase is related to the relative phase between E and H). In the far field electromagnetic radiation resembles a plane wave at least locally. For instance, vacuum's characteristic impedance is approximately 377 ohms. This way, and EM wave propagating in vacuum having an electric field of 377 V/m will have a magnetic field of amplitude 1 A/m. All units in the international system. The "strength" comparison between the value of the electric field and the magnetic field is meaningless, since they are measured in distinct units, E in V/m and H in A/m. What is stronger, as a matter of fact, is the force exerted by the eletric field in a point charge since it is Fe = q E. The magnetic force is Fm = q v x B which is of the order of (v/c) Fe. For non relativistic particles v
The answer to these questions goes as follows, if I correctly understood it: for an electromagnetic plane wave the ratio between the electric and magnetid field is a constant of the given medium, known as the characteristic impedance of the medium (the modulus of it is the ratio between E and H, while the phase is related to the relative phase between E and H). In the far field electromagnetic radiation resembles a plane wave at least locally. For instance, vacuum's characteristic impedance is approximately 377 ohms. This way, and EM wave propagating in vacuum having an electric field of 377 V/m will have a magnetic field of amplitude 1 A/m. All units in the international system. The "strength" comparison between the value of the electric field and the magnetic field is meaningless, since they are measured in distinct units, E in V/m and H in A/m. What is stronger, as a matter of fact, is the force exerted by the eletric field in a point charge since it is Fe = q E. The magnetic force is Fm = q v x B which is of the order of (v/c) Fe. For non relativistic particles v
In a travelling wave energy constantly goes from electric form into magnetic and back. So the mean (in time) electric energy in any point equals that of magnetic energy. Thus one have $\mu_0 H^2 = \epsilon_0 E^2$. From this formula you also get the "impedance" relation for the fields strength: $Z_0 H = E$, where $Z_0 = \sqrt{\mu_0 / \epsilon_0}$ is the "wave impeadance"
Referring to the manner in which an electron can be accelerated in straight line by submitting it to 2 equal intensity electric and magnetic fields (v=E/B), I see a parallel with plane wave treatment of free electromagnetic energy, which by definition represents the wavefront point considered as moving in straight line (along the x axis), it would seem indeed that both fields have to be of equal intensity (c=E/B).
It illustrates the characteristic impedance of your medium, it's like the R=V/I relationship in transmission line theory
best regards
The ratio for the electric and magnetic field for an electromagnetic wave is dependent upon how far from the source you are. If in the far-field it will be approx 377 ohms, otherwise, when you are in the near-field zone it will be greatly dependent upon the distance to the source.
I think, ratio of electric component to magnetic component is equal to the ratio of permittivity to permeability of medium of EM radiation.
I think the key is in defining the word "strength". If we go with force it creates a problem with the measurement (as explained in César Augusto Dartora's post).
Could we instead define strength in terms of instantaneous energy density? I expect the calculations for working out energy density of the E and H components would follow from the usual derivation of Poynting's theorem. For a plane wave in free space this should (I could be wrong) work out to both components having same energy density and hence the same capacity for work. For other propagation modes and other mediums this would be different for each case due to the different eps_r and mu_r and also the variance in field distribution. The total time average energy in each case must of course remain the same (assuming the lossless case).
Fundamentally, Ratio of the two fields are nothing but it is ratio of voltage to current. Electric field is caused by voltage and magnetic field is caused by current. In the form of electrical energy or energy flowing through wired media is in the form of voltage and current. Now how much current flow in a a circuit is depend on applied voltage and offered impedance at the point.
EM wave is nothing but electric energy conversion into electromagnetic wave energy. Electric field represents voltage and Magnetic field represents current. Thus, ratio is an impedance of media.
The impedance of media is sqrt (ratio of permittivity to permeability) or 120*PI * sqrt(ratio of relative permittivity to relative permeabillity)
If characteristic impedance and one of the field is known, one can calculate the other field
When propagating in a medium, this ration is known as impedance (a tensor), which depends on the medium electric conductivity and /or dielectric permittivity, and magnetic permeability, as well as on the frequency.
In technical practice, we use this table: Attached in annex
In addition: Electric field strength: E = H × Z0, (Z0 ≈ 377 Ω ≈ 120*PI)
I hope I helped a bit
1. Maxwell's equations are symmetric for the electric and magnetic fields in a homogeneous isotropic medium. This is the equivalence of fields.
2. An arbitrary electromagnetic field in such an environment can be represented as a superposition of purely transverse waves: plane, spherical, or cylindrical. The ratio of the strengths of electric and magnetic fields is equal to sqrt(mu/eps) for all purely transverse waves, i.e. it depends on properties of the medium.
3. The electric field plays a greater role than the magnetic field in the daily lives of people. Why? It is a matter for another discussion.
Best regards.
The exact ratio will depend upon how far the transmitter and receiver are.
As stated above if they are far away the meassurment is done in the far-field and the ration will be approx 120*pi = 377 ohm otherwise the ration will oscillate depending upon the exact distance. See here for a discussion on near and far field: https://en.wikipedia.org/wiki/Near_and_far_field
http://www.williamson-labs.com/images/near-far.gif
The wikipedia page is referring to the electromagnetic field around macroscopic objects such as a transmitting antena not about free EM ambient radiation in vacuum, which is the resultant of the combined emission by de-exiting electrons as they drop to least action orbitals in atoms all over the universe, if I understood correctly Andrew's initial question.
If such is the case, it must be taken into account that each individual "wave" emitted (if the wave concept is taken as physically occurring) by each such de-exciting electron is emitted from a very localized "point-like" source and can then only be in spherical expansion away from this point. The same "local emission circumstance" would also apply to any nuclear fusion or fission emission.
In such cases, far field behavior seem to be the only option if plane wave treatment is applied to any point of the expanding wave front of any such occurrence.
Andrew, Hi!
If I understand Your questions, they are related to classical electrodynamics. A vacuum is an empty space for it. My answers 1 and 2 are fair for empty space. Physical or QED vacuum is an object of another theory.
Sincerely, Jacob.
The well known monotonic E-H relationship (i.e., in terms of a fixed value for a medium impedance) holds for sure for a) free-space AND b) assuming a certain distance away from the source (far-field region). This is the case because the underlying typical assumption is "plane wave condition".
Even in free-space, but in near- and intermediate-field zones, the relation is not monotonic and its derivation comes directly from non-simplified solutions of the Maxwell equations and also by considering the geometry of the source antenna. For simple antenna cases and short distances, these solutions are well known (for example, check the topic "Near Field Communication", or NFC).
If the scenario is not free-space, even considering homogeneous and isotropic medium, the problem is more complex because there are also circuitry inefficiencies that may not be captured by a single-instance application of the Maxwell equations. For example, aspects such as if the antenna is insulated or not may impact the solution (for further investigations, check these two books from Ronold King: "Antennas in Matter" and "Lateral Electromagnetic Waves: Theory and Applications to Comm. Geophysical Exploration, and Remote Sensing"). Nonetheless, for this non-free-space scenario, the E-H relationship will eventually become monotonic after a certain distance from the source and it can also be expressed in terms of medium impedance (similar to Zo for the free-space case).
All the above discussions assume that the medium is unbounded, that is, the transmitting and receiving antennas are not relatively close to an interface between two different materials. If it is not the case (i.e., one or both antennas are electrically close to an interface), the solution of the Maxwell equations still can be applied but the radiating energy will be split into different "kinds" of waves. Usually, we think about this scenario by means of an optical-style analogy involving reflected and refracted waves. However, if more kind of waves exist (such as "surface waves"), it is necessary to confirm if they do not impact the initial analysis. If these waves have significant energy, the application of a generic impedance ratio for medium 1 and 2 may be distorted. The electrical and magnetic properties of both media are associated with how strong these "erratic" wave modes are. Nonetheless, one can still limit the analysis to a single, main (energy dominant) wave mode thus neglecting the other modes - in this scenario, the conventional E-H relationship can be considered acknowledging the existence of multiple waves in that scenario.
Finally, when the medium cannot be approximated as isotropic and homogeneous one, the Maxwell equations must be solved in many distinct smaller sub-domains of the problem and usually numerical methods and software solvers are used to this end. In such cases, the concept of a medium impedance Z to represent the E-H relationship for the overall medium may be too strong, even at the far-field region, although can it can still be applied regarding distinct locus in space.
Dear Sirs!
Remember, please, in a homogeneous isotropic medium in the absence of gravity, the photons move in straight lines. They create electric and magnetic fields perpendicular to the direction of movement. Discussion of near-field and different types of waves in waveguide structures is not appropriate, because they are formed from a mixture of photons moving in different directions.
Sincerely, Jacob Reyzenkind.
Perfectly ok to compare apples and oranges.
1.) the "equivalency" is in Lorentz F = q (E + v X B), as an electron e, when E=0, F=evB (as in CRT plates) ... likewise when B=0, F=eE (CRT gun). Now get your "equivalency" by setting the apple force to the orange force.
2.) their exact ratio is then E/vB. So if you're moving fast the exact ratio is small, and if moving slow the exact ratio is large.
Can you think of a situation whereby -e speed yields an exact ratio of 1?
Dear Henry,
Yes we can.
We get a ratio of 1 for E and B fields "energy densities" between both fields, in all cases where an electric charge (an electron, for example) is made to move in straight line according to v=E/B:
See equation (35) in this paper, where you will find how to calculate the ambient E and B fields of the carrying energy that causes an individual electron to move in physical reality:
http://www.gsjournal.net/Science-Journals/Research%20Papers-Relativity%20Theory/Download/2257
Best Regards
André
The electric field is not stronger than the magnetic field. They have different units and the ratio between them depends entirely on the units you use. The magnetic field is exactly equal to that caused by the changing electric field around it and the electric field is exactly equal to that caused by the changing magnetic field around it.
When two waves that are going in opposite directions cross each other, a standing wave pattern is formed in which the same energy is alternately stored as magnetic energy and then electric energy. In this measurement, of energy with the same units for each, both fields have the same strength.
Hi Malcom:
While semantically I could agree with your statement if you were to pick a particular condition (e.g. propagation in free space), I think you're missing the fact that the electric and magnetic field are related by the impedance of the propagation medium. Thus, with the 377 Ohm impedance of free space, we could define the electric and magnetic fields to be "equivalent" but then when I move to a 50 Ohm system, by definition the fields would no longer be equivalent. Since we defined E = H' when E/H = 377 Ohms (H' = 377 H), then when we change to a system where E/H = 50 Ohms, then one of the two of E or H' (or both) must have changed. Now granted, we could potentially define H' as Z0 H or E' as E/Z0, but those numbers no longer have an equivalent meaning in the different propagation environments. We would have embedded the definition of the environment into the field, preventing us from talking about equivalent electric or magnetic fields in those two different environments. That doesn't work very well.
Michael
When energy is considered, the impedance makes no difference. The same amount of energy is represented by both fields whatever the impedance.
The strength of the two fields depends entirely on the units chosen. We choose to use impedance in ohms, but that does not mean the E field is 377 times more than the H field when the impedance is 377 ohms. It is 377 volts per amp. Likewise when the impedance is 50 ohms.
The OP was asking about field strengths, not the energy in a field or power in a wave. Charges and currents react proportional to field strengths, not energies (field strength squared). So while yes, the energy is carried equally between the electric and magnetic components, that includes the corresponding effect of epsilon and mu (permittivity and permeability). The corresponding E and H fields (not energy/power) are NOT equivalent.
@michael
To what extent are they not equivalent 0%, 2%, 1%, 10%, 20 %,
Maxwell has: div B =0, div E= p/e0, so is magnetism nothing.
André,
Goodbye to thread focus and hello to 1st paragraph of Abstract. Any idea what’s actually being integrated out to infinity? (rhetorical question). It’s surely not “their” energy fields. Isn’t anyone even curious to know what’s magically being accounted for in that integral? …if like most, you don’t care because while the assumption is insane to any approximation, it makes the math work. Yet we always ignore the special sauce.
Back to velocity. The “velocity” in question is a resultant superposition of many other component-velocities (spin, precession, etc) of that same electron that do not cancel out. The straight line motion is a surprising special case of cancellations, I didn’t know so thanks for pointing that out. But I do know that electron velocity is the result of complications in their interactions with their surroundings, and at times can be surprising …for example, would you be surprised if the integral of velocity superpositions for bounded electrons of varying degrees and their special sauce interaction gave rise to gravitons?
Dear Henry,
You write: "Any idea what’s actually being integrated out to infinity? (rhetorical question). It’s surely not “their” energy fields. "
I think the question is not rhetorical at all. I always drew a very distinct line between mathematics as a language and what this language is meant to describe.
So fundamentally, what I see is this: The "integral", as we name this mathematical tool, does not "integrate" anything. It simply "calculates" an amount of energy (or force, depending on what we ask it to do) as a function of the inverse of the distance, because this is what we parameterize it to do.
As for "fields", they also are mathematical tools, but tools that allow graphically "representing" or "visualizing" the intensity of either the force as a function of distance, or the intensity of energy.
So I see no magic at play at all, simply applying a metaphorical "abstract ruler" to measure force and/or energy, no more mysterious to me than applying an actual straight edge ruler to measure the length of sheet of paper.
If you have a look at Figure 1 of this recently published paper (In section 6, page 1064), you will see how the integral tool can be compared to the straightforward use of the Coulomb equation, that does not use the "integration tool":
http://file.scirp.org/pdf/JMP_2018042716061246.pdf
What exists out there at the submicroscopic level is localized point-like behaving elementary particles, whose "substance" seems to be "energy", and whose physical existence can be confirmed via scattering experiments, and whose rebound trajectories can be made visible by them ionizing water vapour bubbles in bubble chambers as they whiz through the chamber for example.
As for the velocity of the electron, it can be expressed longitudinally inasmuch as it is not kept captive in some axial resonance state in some orbital or other.
Like you, the special case of straight line motion of an electron due to transversally cancelling E and B "resultant force" interaction when both "fields" representing equal carrying energy densities always fascinated me from the first time I succeeded in actually visualizing its vectorial representation.
But it would more than surprise me if the integral of velocity superposition for bounded electrons could give rise to gravitons. From my perspective, gravitation would not be mediated via gravitons, but via a general underlying gradient related to Coulomb interaction:
https://www.omicsonline.org/open-access/gravitation-quantum-mechanics-and-the-least-action-electromagneticequilibrium-states-2329-6542-1000152.pdf
Best Regards
André
Dear sirs! Do you remember the questions asked by Mr. Andrew Worsley?
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